1. Field of the Invention
The present invention relates to a dynamic-quantity sensor for sensing a dynamic quantity, such as acceleration, angular acceleration, angular velocity, or load.
2. Description of the Related Art
The present applicant has filed Japanese Unexamined Patent Application Publication 2002-243757 (Patent Document 1) and Japanese Unexamined Patent Application Publication 2003-254991 (Patent Document 2) regarding a dynamic-quantity sensor with piezoelectric vibrators.
The configuration of an acceleration sensor in Patent Document 1 is shown in FIG. 13 herein. As shown in FIG. 13, a bridge circuit 110 includes two piezoelectric vibrators S1 and S2 to which stresses induced by acceleration are applied in opposite directions, and capacitors C1 and C2. A voltage-dividing impedance circuit 120 is disposed between nodes p2 and p3. A signal at a voltage division point p5 of the voltage-dividing impedance circuit 120 is fed back to a node p1 by a feedback signal processing circuit 130. In this manner, an oscillator circuit is provided. An oscillation-output phase difference between the nodes p2 and p3 is detected by a phase-difference signal processing circuit 140, and the phase difference is output as an acceleration detection signal.
The configuration of a dynamic-quantity sensor in Patent Document 2 is shown in FIG. 14 herein. Two piezoelectric vibrators Sa and Sb are arranged so that stresses induced by a dynamic quantity, such as acceleration, are applied thereto in opposite directions. A current-to-voltage-converting/signal adding circuit 11 converts current signals flowing through the two piezoelectric vibrators Sa and Sb into voltage signals. A voltage-amplifying/amplitude-limiting circuit 12 amplifies a combined signal Sab of the two voltage signals to generate a voltage signal Vosc, which is in phase with the current signals, and positively feeds back the voltage signal Vosc to cause oscillation. A phase-difference-to-voltage converter circuit 13 generates a voltage signal in proportion to a phase difference between the voltage-converted signals Sa and Sb. An amplifying/filtering circuit 14 performs direct-current (DC) amplification of the voltage signal and removes unnecessary frequency components from the voltage signal.
As uses and requirements of dynamic-quantity sensors for sensing a dynamic quantity, such as acceleration, have increased and become more varied, it has been strongly demanded to reduce the cost thereof. However, the acceleration sensor in Patent Document 1 requires many components since two pairs of circuits are required to detect a phase difference, and an adding circuit (averaging circuit) including a voltage-dividing impedance circuit is necessary to determine the oscillation frequency. It is thus difficult to reduce the size and cost of such an acceleration sensor. Similarly, the dynamic-quantity sensor in Patent Document 2 requires relatively large-scale circuits, such as the current-to-voltage-converting/signal adding circuit and the voltage-amplifying/amplitude-limiting circuit, for driving two piezoelectric vibrators in parallel with each other, and it is thus difficult to reduce the size and cost of such a dynamic-quantity sensor.